Promote @FlowPreview API to stable/experimental (#3548)

* Promote @FlowPreview-marked API to stable
* Promote @FlowPreview to @experimental where appropriate
* Delay-related operators are not promoted as they are clearly underdesigned
* Promote AbstractFlow
* Promote flat* related operators to gather more feedback and to work on them incrementally later in 1.7+

Fixes #3542
Fixes #3097

kotlinx-coroutines-core/common/src/flow/Builders.kt

@@ -65,7 +65,6 @@ private class SafeFlow<T>(private val block: suspend FlowCollector<T>.() -> Unit
 /**
  * Creates a _cold_ flow that produces a single value from the given functional type.
  */
-@FlowPreview
 public fun <T> (() -> T).asFlow(): Flow<T> = flow {
     emit(invoke())
 }
@@ -80,7 +79,6 @@ public fun <T> (() -> T).asFlow(): Flow<T> = flow {
  * fun remoteCallFlow(): Flow<R> = ::remoteCall.asFlow()
  * ```
  */
-@FlowPreview
 public fun <T> (suspend () -> T).asFlow(): Flow<T> = flow {
     emit(invoke())
 }

kotlinx-coroutines-core/common/src/flow/Channels.kt

@@ -168,7 +168,6 @@ public fun <T> BroadcastChannel<T>.asFlow(): Flow<T> = flow {
  * default and to control what happens when data is produced faster than it is consumed,
  * that is to control backpressure behavior.
  */
-@FlowPreview
 public fun <T> Flow<T>.produceIn(
     scope: CoroutineScope
 ): ReceiveChannel<T> =

kotlinx-coroutines-core/common/src/flow/Flow.kt

@@ -221,7 +221,7 @@ public interface Flow<out T> {
  * }
  * ```
  */
-@FlowPreview
+@ExperimentalCoroutinesApi
 public abstract class AbstractFlow<T> : Flow<T>, CancellableFlow<T> {
 
     public final override suspend fun collect(collector: FlowCollector<T>) {

kotlinx-coroutines-core/common/src/flow/operators/Merge.kt

@@ -17,16 +17,19 @@ import kotlinx.coroutines.flow.internal.unsafeFlow as flow
 
 /**
  * Name of the property that defines the value of [DEFAULT_CONCURRENCY].
+ * This is a preview API and can be changed in a backwards-incompatible manner within a single release.
  */
 @FlowPreview
 public const val DEFAULT_CONCURRENCY_PROPERTY_NAME: String = "kotlinx.coroutines.flow.defaultConcurrency"
 
 /**
  * Default concurrency limit that is used by [flattenMerge] and [flatMapMerge] operators.
  * It is 16 by default and can be changed on JVM using [DEFAULT_CONCURRENCY_PROPERTY_NAME] property.
+ * This is a preview API and can be changed in a backwards-incompatible manner within a single release.
  */
 @FlowPreview
-public val DEFAULT_CONCURRENCY: Int = systemProp(DEFAULT_CONCURRENCY_PROPERTY_NAME,
+public val DEFAULT_CONCURRENCY: Int = systemProp(
+    DEFAULT_CONCURRENCY_PROPERTY_NAME,
     16, 1, Int.MAX_VALUE
 )
 
@@ -39,7 +42,7 @@ public val DEFAULT_CONCURRENCY: Int = systemProp(DEFAULT_CONCURRENCY_PROPERTY_NA
  * Note that even though this operator looks very familiar, we discourage its usage in a regular application-specific flows.
  * Most likely, suspending operation in [map] operator will be sufficient and linear transformations are much easier to reason about.
  */
-@FlowPreview
+@ExperimentalCoroutinesApi
 public fun <T, R> Flow<T>.flatMapConcat(transform: suspend (value: T) -> Flow<R>): Flow<R> =
     map(transform).flattenConcat()
 
@@ -63,7 +66,7 @@ public fun <T, R> Flow<T>.flatMapConcat(transform: suspend (value: T) -> Flow<R>
  * @param concurrency controls the number of in-flight flows, at most [concurrency] flows are collected
  * at the same time. By default, it is equal to [DEFAULT_CONCURRENCY].
  */
-@FlowPreview
+@ExperimentalCoroutinesApi
 public fun <T, R> Flow<T>.flatMapMerge(
     concurrency: Int = DEFAULT_CONCURRENCY,
     transform: suspend (value: T) -> Flow<R>
@@ -75,7 +78,7 @@ public fun <T, R> Flow<T>.flatMapMerge(
  *
  * Inner flows are collected by this operator *sequentially*.
  */
-@FlowPreview
+@ExperimentalCoroutinesApi
 public fun <T> Flow<Flow<T>>.flattenConcat(): Flow<T> = flow {
     collect { value -> emitAll(value) }
 }
@@ -132,7 +135,7 @@ public fun <T> merge(vararg flows: Flow<T>): Flow<T> = flows.asIterable().merge(
  * @param concurrency controls the number of in-flight flows, at most [concurrency] flows are collected
  * at the same time. By default, it is equal to [DEFAULT_CONCURRENCY].
  */
-@FlowPreview
+@ExperimentalCoroutinesApi
 public fun <T> Flow<Flow<T>>.flattenMerge(concurrency: Int = DEFAULT_CONCURRENCY): Flow<T> {
     require(concurrency > 0) { "Expected positive concurrency level, but had $concurrency" }
     return if (concurrency == 1) flattenConcat() else ChannelFlowMerge(this, concurrency)

reactive/kotlinx-coroutines-reactive/src/Migration.kt

@@ -30,7 +30,6 @@ public fun <T : Any> Publisher<T>.asFlowDeprecated(): Flow<T> = asFlow()
 public fun <T : Any> Flow<T>.asPublisherDeprecated(): Publisher<T> = asPublisher()
 
 /** @suppress */
-@FlowPreview
 @Deprecated(
     message = "batchSize parameter is deprecated, use .buffer() instead to control the backpressure",
     level = DeprecationLevel.HIDDEN,